Radar ranges are used for measuring radar cross section (RCS) of platforms and for testing antennas. In a typical radar range, a target is placed in a target zone at a finite distance from a radar source where wave fronts appear flat (that is, a far field condition). For an outdoor (or “free space”) range, this distance can be on the order of around 10–15 miles or so.
A desire to reduce the size of radar ranges from the size of outdoor ranges has led to the development of compact indoor ranges (or “compact ranges”). A typical compact indoor range presents in hundreds of feet the same amplitude and phase conditions that an outdoor (free space) range presents in 10–15 miles. Typical compact ranges use frequencies around 1 GHz and above.
In a compact range, an antenna transmits radar waves toward a curved reflector. The incident radar waves induce local perturbations of current on the surface of the curved reflector. The local perturbations of current generate radar waves with flat wave fronts (that is, a flat field) that are emitted from the curved reflector. However, at a finite distance from the curved reflector in a conventional compact range, the flat field diverges due to Huygens principle and begins to appear like a spherical wavefront.
The distance at which the divergence begins is a function of frequency. The test zone field begins to transition from flat to spherical when the diameter of the curved reflector is less than around 5–10 times the wavelength λ of the radar waves. Therefore, as frequency decreases, the point at which divergence begins gets closer to the reflector.
For some testing applications, it is desirable to use frequencies, such as around 100 MHz or so, that are lower than those frequencies typically used in conventional compact ranges. However, at some frequency, the flat field could possibly diverge to spherical wavefronts even before the wavefronts arrive at the target zone. Therefore, there is a lower limit to frequency at which far-field conditions can be produced at a fixed target zone in a conventional compact range.
The local perturbations of current are induced over a surface of the curved reflector that spans approximately one-half the radius from the center of the reflector. It is undesirable to use outer surfaces of the curved reflector because diffraction from the edges of the reflector produces ripples in the test zone. Various treatments for limiting the diffraction include serrations, graded-resistive cards, and rolled edges. However, depending on the size of the treatment, the outer edge of the reflector may be kept from contributing to the test zone fields.
As a result, useable size of the reflector may be reduced. Because the transition of the test zone field from flat to spherical begins when the diameter of the curved reflector is less than around 5–10 times the wavelength λ, the reduction in the useable size of the reflector corresponds to an increase in the lowest frequency of operation.
Dual reflector systems have been employed to provide control over illumination in the test zone. A sub-reflector is used to accurately map a feed pattern onto a main curved reflector. Some reduction in illumination of the main reflector edges may result, thereby providing the same diffraction-reduction benefits as the treatments discussed above. This results in a small increase in useable size of the main reflector (over direct feed without the sub-reflector) and a corresponding decrease in the lowest frequency of operation.
Conventional compact ranges have been considered unusable below the lowest frequency achievable with a sub-reflector. As a result, testing below such frequencies has been performed at outdoor (free space) ranges at high cost.
Another approach to testing applications considered too low for conventional (that is, reflector-only) compact ranges uses a phased array in the compact range instead of the curved reflector. In one example, a two-dimensional (16 element×16 element) phased array about the same size as the curved reflector is placed in a compact range in front of the curved reflector. Amplitude and phase of the elements of the phased array are adjusted such that wave fronts are flat at the target zone.
To use the phased array at relatively high frequencies such as around 400 Mhz in a compact range, the two-dimensional array would have to include thousands of elements. The costs for such an array are not feasible and useable bandwidth is limited to around one-half octave. Thus, such a phased array is not a viable option at such relatively high frequencies. Instead, use of the phased array is limited to frequencies considered too low for the curved reflector.
The phased array must be positioned in place in front of the curved reflector for use. Likewise, the phased array must be removed from its position in front of the curved reflector when the phased array is not in use. As a result, time and labor costs are introduced for setting up, aligning, and storing the phased array. Also, facility resources are required for storage and transport of the phased array.
Therefore, it would be desirable to create a flat field in a compact range with a curved reflector, and without use of a separate phased array of elements, at frequencies lower than those that are possible with currently known, reflector-only compact ranges.